1. Technical Field
The present technology pertains generally to magnetoelectric heterostructures and more particularly to devices and methods that manipulate magnetic anisotropy of magnetic materials using highly localized biaxial strain in a piezoelectric substrate with patterned electrodes. Since only a small region surrounding the electrodes is strained, arrays of indexed magnetic elements can be created.
2. Background
The manipulation of magnetization at the micro- and nanoscale levels has been studied extensively for use in next generation computer memory, nanoscale sensors, and spintronic devices. Several approaches at controlling magnetization have been attempted including magnetic fields, spin-polarized current injection, exchange-bias, interface-charge-driven magnetoelectric (ME) effect, strain mediated ME effect, and ferroelectric/ferromagnetic coupling in single phase multiferroics, with limited success.
The use of a strain-mediated approach to control magnetization is attracting attention due to the promise of low energy consumption, large coupling coefficients, and the wide availability of piezoelectric-magnetoelastic materials.
For the past decade, researchers have focused on developing a magnetic memory element and other devices using a multiferroic material systems. One approach uses a strain-mediated composite consisting of layered piezoelectric plate with top and bottom electrodes and magnetoelastic materials strain coupled together at the interface. However, in this configuration the entire piezoelectric (or ferroelectric) plate is subjected to the same strain. The magnetism of individual magnetic structures cannot be independently controlled. Existing fabrication technology enables the development of layered structures in which a thin piezoelectric (or ferroelectric) layer can be deposited on a thicker substrate, e.g. PZT on Si. In this configuration with the top and bottom of the piezoelectric layer electrodes, the thicker substrate clamps the in-plane strain components.
Using the example of a multiferroic composite memory element, the multiferroic composite memory element is typically fabricated on a fairly thick substrate system, e.g., silicon. Substrate clamping is a significant issue. This thick substrate clamps the piezoelectric/magnetoelastic material, limiting the amount of strain that can be generated, posing a significant challenge for the implementation of controlling the magnetization of individual magnetic features such as a strain-mediated memory element. Bulk piezoelectric substrates require high voltage, are semiconductor incompatible, and are rate limited by elastic wave velocities through the thickness. Thin film piezoelectrics are also clamped by the thick substrate that prevents strain transfer limits in-plane strain.
Another consequence of the use of a monolithic piezoelectric substrate with the entire surface electrode is that all magnetic surface elements are subjected to the same strain field and would rotate or switch all of the elements simultaneously. The development of devices such as a strain-mediated multiferroic memory device requires the magnetization of each element to be individually controllable using a piezoelectric (or ferroelectric) thin film grown on a substrate such as a Si/SiO2 wafer.
Achieving local control of individual magnetoelectric structures on a piezoelectric thin film is also a challenging problem. Piezoelectric thin films that are fully clamped in-plane by the substrate dramatically reduce the effective piezoelectric coefficient and restrict its capability of generating enough strain for controlling magnetization. Further, generating in-plane strain in a piezoelectric thin film on a length scale comparable to the substrate thickness typically requires bending of the entire substrate which is impractical.
Accordingly, a method is needed for controlling the magnetic behavior of individual magnetic islands features on the surface of a thin film piezoelectric using a strain mediated approach. The present technology satisfies this need and provides devices and methods for reorienting a single domain structure magnetization controlling magnetic behavior in a magnetoelastic surface feature deterministically using a strain-mediated approach by controlling strain in highly localized regions of a piezoelectric (or ferroelectric) layer that is fully clamped to a substrate.